The field of the invention is industrial control systems such as process controls, numerical controls, and programmable controllers, and particularly, to redundant local area networks for connecting such controls together.
Local area networks are comprised of two or more nodes, or stations, which are linked together by a communications media. The communication media may take a number of forms, including coaxial cable, fiber optics or twisted wire pairs. The topology of the links between stations may also take a number of forms, including star, multidrop or ring configurations.
Regardless of the media used or the topology of the network, a control scheme is required to provide an orderly transfer of information from one station to another on the network. The most primitive schemes employ a master station which is responsible for controlling the operation of the network. The master may "poll" each slave station on the network for information to be delivered to other stations, or it may enable a polled station to transmit its message directly to other stations. Such networks are not appropriate for industrial applications because the entire network is brought down if a malfunction should occur in the master station.
Another solution is to employ a carrier sense multiple access (CSMA) scheme such as that defined by the Ethernet standards. In CSMA networks each station waits for the network to go silent, and if it has a message to send, it takes control of the network and begins transmission. A mechanism must be provided to detect "collisions" when two or more stations transmit messages at the same time, and for contending with those stations to determine which has priority. While CSMA networks are appropriate for office and commercial applications, they are not appropriate for industrial networks which convey information required by "real time" control systems. Other, more "deterministic" network control schemes are required for industrial networks.
Other, more deterministic network control schemes include time slot reservation, slotted rings, register insertion rings and token passing networks. With a token passing scheme, a token in the form of a distinctive bit pattern is passed between stations on the network. While a station has the token it can transmit messages to other stations or command other stations to transmit messages. While token passing schemes are simple in concept, they are complicated to implement due to the problem of lost tokens and duplicate tokens. These problems are more severe in the industrial environment where power may be lost, where stations may be added or removed from the network, or where malfunctions may occur in a station or the media connecting the stations. U.S. patent application Ser. No. 771,834 filed on Sept. 3, 1985, and assigned to same assignee as the present application, teaches one type of a token passing network wherein each node of the network assumes control of the maintenance of the network while it possesses the network master token.
Several problems are encountered with local area networks of whichever type when they are used to control various pieces of production equipment on an assembly line. One problem is that the various manufacturing processes, such as electric welding, may induce noise into the electrical cable for the network. This noise may be erroneously interpreted as a valid message or may garble messages. Another problem may be the physical interruption of the cable by accidential severing or by the malfunction of a piece of equipment that is connected to the network. In order to overcome these problems, various types of survivable communication systems have been devised. One such example is shown in U.S. Pat. No. 4,575,842 issued on Mar. 11, 1986 to Joseph L. Katz, et al. This network utilizes a number of separate cables so that when a fault is encountered in one of the cables that one may be isolated and another cable used in its place. The signal is typically transmitted over only one cable and various devices on the network switch the signal between various cables in order to patch around a faulty segment.
Another redundancy technique that was considered by the present inventors is the use of two separate communication media and transmitting each message over both media. Each station then receives both messages and decodes them. The received data from each media is checked using a frame check sequence that is part of the message. The more reliable data is then ueed by the station. This technique, although relatively simple to implement requires two complete data receivers to convert both encoded network transmissions into the digital information. This significantly increases the cost of each station.